1. Field of the Invention
The present invention relates to ceramic-metal composite materials, or cermets, and methods of producing such composites. More particularly, the present invention relates to ceramic-metal composites having graded properties resulting from a change in the ratio of ceramic to metal in the material and the methods for producing such composites.
2. Description of Related Art
During the last few decades, ceramics have been investigated for use in many structural applications, particularly in high temperature environments. However, ceramic materials are not always well suited since they are brittle, have a limited ductility and low values of fracture toughness at low temperatures. In addition, the fracture strength of ceramics is not very reproducible since the average strength usually varies from one lot of parts to the next, which is attributed to the presence of processing flaws which can initiate fractures. A great deal of effort has been expended in an attempt to increase the fracture reliability of ceramic materials and to develop tough and creep-resistant ceramic composites.
One possible solution is the fabrication of a ceramic-metal composite, also commonly referred to as a cermet. Traditionally, ceramic-metal composites have been produced in one of two ways; (1) by heating mixtures of ceramic and metal materials to obtain a metal matrix having a discrete ceramic phase, or (2) as disclosed in U.S. Pat. No. 2,612,443 by Goetzel at al., issued Sep. 30, 1952, by forming a sintered porous body that can be a metal, metal-carbide or metal-nitride, and infiltrating the porous body with a molten metal by the use of mechanical squeeze-casting or other means of applying pressure to force the molten metal into the voids within the porous body.
Other approaches for forming cermets have been used due to a lack of success in obtaining adequate control of cermet composition and form with traditional processes. For example, use of accelerated oxidation reactions and "combustion wave" exothermic reaction processes have been utilized to form cermets. See, for example, U.S. Pat. No. 4,988,645 by Holt et al., issued Jan. 29, 1991.
The LANXIDE process, such as that disclosed in U.S. Pat. No. 4,853,352 by Newkirk et al., issued Aug. 1, 1989, relates to a method for forming cermets whereby a molten parent metal is oxidized, usually in the presence of oxidation enhancing dopants, to create a three-dimensional interconnected ceramic-metal composite material which contains between 1% and 40% of the parent metal by volume. This process is limited in that only the parent metal is infiltrated into the oxide reaction product and the process takes extended periods of time, such as 48 hours or more.
Infiltration of molten metals into porous ceramic preforms by squeeze casting and by applying pressure to the molten metal is known, for example, see Verma and Dorcic, "Performance Characteristics of Metal-Ceramic Composites Made by the Squeeze Casting Process", Ceramic Engineering Science Proc., Vol. 9, pp. 579-596 (1988). However, it is difficult to achieve near complete infiltration of the void space within the preforms without use of substantial pressure. In addition, when ceramic preform materials contain a high volume of porosity, the use of pressure in squeeze casting techniques can crumble the ceramic structure. The use of pressure can also preclude the formation of ceramic-metal composites having complex shapes. Further, these processes require complex pressure dies and frequently require extensive flash removal, that is, removal of excess metal.
Infiltration using vacuum furnaces and using infiltration enhancers are also described in the art. U.S. Pat. No. 3,864,154 by Gazza et al., issued Feb. 4, 1975, discloses a method for the infiltration of aluminum or silicon into a cold-pressed compact of boron-containing ceramics (e.g., aluminum boride or silicon boride) in a vacuum furnace. It is disclosed that the infiltration process takes about 2 hours.
U.S. Pat. No. 4,828,008 by White et al. issued on May 9, 1989. White et al. disclose a method for infiltrating aluminum alloys into a permeable mass of loose ceramic powder, such as alumina. A nitrogen gas atmosphere must be used and magnesium must be alloyed into the aluminum metal to achieve spontaneous infiltration. U.S. Pat. No. 5,016,703 by Aghajanian et al. and issued on May 21, 1991, discloses a process for the spontaneous infiltration of aluminum into a ceramic preform that comprises a mass of particles, platelets, whiskers or fibers. An infiltration enhancer, such as magnesium turnings, is placed between the molten metal and the preform to enhance the infiltration. The infiltration time is on the order of about 5 hours.
U.S. Pat. No. 5,004,035 by Burke et al. issued Apr. 2, 1991, discloses the use of infiltration enhancers for infiltrating aluminum alloys into alumina or silicon carbide preforms that comprise loose particles of materials such as alumina or silicon carbide. After infiltration, which can take on the order of about 10 hours, the metal composite can be reheated and worked to vary the properties of the composite.
U.S. Pat. No. 5,017,533 by Newkirk et al. issued on May 21, 1991. Newkirk et al. disclose a method for producing a self-supporting ceramic body by oxidation of a molten precursor metal with a vapor-phase oxidant to form an oxidation reaction product. A second metal is incorporated into the molten flux during the oxidation reaction. For example, copper can be alloyed into aluminum which is then oxidized to form an alumina oxidation product. The oxidation process takes on the order of 48 hours or more.
U.S. Pat. No. 5,007,475 by Kennedy et al. issued on Apr. 16, 1991. Kennedy et al. disclose the formation of a metal matrix composite body by the spontaneous infiltration of a molten matrix metal into a three-dimensional interconnected material. The metal is an aluminum alloy and the three-dimensional matrix is preferably alumina. The aluminum alloy is placed on top of the three-dimensional interconnected material and the assembly is placed in a containing vessel, which is then heated to infiltrate the metal into the three-dimensionally interconnected material. The typical infiltration time is on the order of about 7 hours or more.
U.S. Pat. No. 4,868,143 by Newkirk et al. and issued on Sep. 19, 1989, discloses a process for making a composite wherein an oxidation reaction product (e.g., alumina) is formed with aluminum parent-metal interconnected therethrough. The composite is then contacted with a second molten metal such as copper or nickel which infiltrates the interconnected parent metal by interdiffusion. The resulting product is a composite having a mixture of two metals interconnected throughout the composite.
U.S. Pat. No. 5,267,601 by Dwivedi, issued on Dec. 7, 1993, discloses a process wherein a permeable mass is formed into a preform having a cavity. The preform containing the cavity is placed at least partially into the molten matrix metal such that an infiltrating atmosphere can communicate with the cavity in order to obtain spontaneous infiltration of the molten matrix metal. Extended periods of time (e.g., 25 to 100 hours) are used to complete infiltration and the preform is only infiltrated to the level that the preform is immersed in the molten metal.
Composites having graded properties have also been suggested in the prior art. For example, see U.S. Pat. No. 3,868,267 by Gazza et al., which issued on Feb. 25, 1975. Gazza et al. disclose a method for the fabrication of a ceramic-metal composite material having a combination of different properties, preferably formed using a gradient system consisting of a transition from a hard, non-ductile front surface to a tough and ductile rear surface. In one embodiment, an AlB.sub.12 powder compact was fabricated wherein one end of the compact had an average particle size greater than the other end of the compact. One end was infiltrated with silicon metal while the other end was infiltrated with aluminum metal. According to another embodiment, one part of a monolithic ceramic compact to be infiltrated includes a ceramic that is wet by a liquid metal and hence is infiltrated by the liquid metal while the other part of the compact consists of a different ceramic that is not wet by the same liquid metal infiltrant and hence is not infiltrated.
U.S. Pat. No. 4,404,262 by Watmough issued on Sep. 13, 1983. Watmough discloses a composite metallic and refractory article in which a metallic layer is partially absorbed within a refractory layer, such a ceramic layer. The density of the refractory ceramic layer increases as it extends away from the metallic layer. It is disclosed that the composite is formed by forcing a molten metal under pressure into the porous structure of the refractory layer.
U.S. Pat. Nos. 4,882,306, 5,164,347, and 5,266,537, all by Kennedy et al., disclose methods for producing a ceramic or ceramic composite body with graded properties. The ceramic is formed by oxidation of a parent metal and the graded properties are obtained by altering the process conditions during the formation of the body by the oxidation reaction. In an embodiment of the invention, it is disclosed that a ceramic matrix was formed from aluminum comprising a zone of alumina and a zone of aluminum nitride by altering the growth atmosphere.
There exists a need for a simple and relatively efficient method for fabricating ceramic-metal composites having graded properties. It would be particularly advantageous if such ceramic-metal composites could be formed using a process that is relatively fast and produces substantially dense and non-porous gradient composites that include substantially continuous metal and ceramic phases, wherein the ratio of metal to ceramic in the article varies through the article.